1. Field of the Invention
The present invention relates to an antenna device preferably for use in a radio frequency identification (RFID) system or in a near field wireless communication system and to a wireless communication device that includes such an antenna device.
2. Description of the Related Art
Generally, for RFID in the 13.56 MHz band such as near field communication (NFC) used for mobile terminals, an RFID chip and a matching device are mounted on a circuit board, an antenna is attached to the inner side of a terminal housing made of resin, and the RFID chip and an antenna are connected via a spring pin or the like in a direct current (DC) manner.
Recent wireless communication devices such as mobile phone terminals have become thinner. In order to cope with insufficient strength due to such thinning, it is increasingly the case that strength is improved by using “more metal” when a housing is made. For example, a resin housing is coated with magnesium or a metal housing made of a material, such as aluminum or carbon fiber, is used.
However, when “more metal” is used to make a housing, an antenna built into a terminal is shielded by the metal. Thus, a problem arises in that communication may not be able to be performed with another device.
As a result, antenna devices have been proposed in Japanese Unexamined Patent Application Publication No. 2011-97657 and Japanese Unexamined Patent Application Publication No. 2011-249935. The antenna devices have a structure in which a metal plate, which has a larger area than an antenna coil, is used as a radiation plate by being arranged to be close to the antenna coil (magnetic-field coupling).
However, the antenna devices described in Japanese Unexamined Patent Application Publication No. 2011-97657 and Japanese Unexamined Patent Application Publication No. 2011-249935 have the following problems.
FIG. 25 is a plan view of an antenna device described in Japanese Unexamined Patent Application Publication No. 2011-97657. A conductor opening CA and a slit 2S are provided in a sheet conductor 2. A coil opening of a coil-shaped conductor 31 is arranged so as to overlap the conductor opening CA. When current flows in the coil-shaped conductor 31 in the directions indicated by arrows drawn with a solid line, a current that flows in the sheet conductor 2 in the directions indicated by arrows drawn with a broken line is induced. In FIG. 25, the current that flows in the coil-shaped conductor 31 and the current that flows in the sheet conductor 2 flow in the same direction in areas A1, A2, A3, and A4. However, the current that flows in the coil-shaped conductor 31 and the current that flows in the sheet conductor 2 flow in opposite directions in an area B. In this manner, when there is an area in which the directions of currents are opposite, the inductance of an antenna (a capacitor) is reduced and communication characteristics are degraded, which is a problem. Moreover, the amount of induced current changes in accordance with positions in which the coil-shaped conductor 31 and the sheet conductor 2 are attached and variations in distance between the coil-shaped conductor 31 and the sheet conductor 2 when they are attached. Thus, variations in inductance tend to occur, which is a problem.